Improved structures for programmable filters: application in a switched-capacitor adaptive filter design

In designing switched-capacitor (SC) filters for high-frequency operation, there is a tradeoff to be made between the gain and the speed of the operational amplifiers (op amps). Taking into account this fact, we present in this paper new structures (an SC integrator and an SC gain stage) which relax the constraint requirements on the op amp while achieving improved performance accuracy. These structures use the double-sampled technique, which increases by a factor of two the maximum speed of operation and correctly operate even with low dc-gain op amps. In order to effectively use all features (low sensitivity to op-amp imperfections) of these SC building blocks in the implementation of an adaptive filter, we also describe an offset-cancelled tunable SC gain stage based on four-quadrant analog multiplier cells. Numerical results are reported to confirm the viability of the proposed design method, and techniques to further improve the adaptive filter electrical characteristics are discussed.